This invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole and more particularly to apparatus and methods for measuring earth formations porosities and borehole washouts or cement voids by means of neutron well logging techniques.
It is well known that oil and gas are more likely to be found in commercially recoverable quantities from those earth formations which are relatively porous or permeable than in more highly consolidated earth formations. Thus, equipment and methods for accurately identifying the porosity of earth formations has substantial industrial importance.
Various methods and apparatus have been proposed in the prior art for utilizing neutron slowing down and diffusion through earth formations to measure porosity. Typically, proposals of this sort have suggested the use of a pressure housing sonde containing a neutron source and a pair of neutron detectors spaced at different distances from the source for transport through a borehole. The thermal neutron detectors utilized in prior art techniques have been used with both pulsed and continuous neutron sources and some combination utilizing the count rate of the detected thermal neutrons has been related to the hydrogen content of the portion of the earth formation being subjected to the flow of neutrons from the neutron source. These methods have generally not proven to be as accurate as desired due to the diameter irregularities of the borehole wall. The variation of the properties of different borehole fluids, the irregular cement annulus surrounding the casing in a cased borehole, and the properties of different types of steel casings and earth formations surrounding the borehole have all tended to obscure the thermal neutron measurements suggested in the prior art.
The thermal neutron population surrounding a source and detector pair sonde as proposed in the prior art can be affected by the chlorine content of the borehole fluid. Similarly other lithological factors such as the boron content of the earth formations surrounding the cased borehole affect thermal neutron populations. Measurements of thermal neutron captures are utilized in neutron lifetime logs or thermal neutron population die away logs of various types as contemplated in the prior art. The present invention, however, rather than relying on a measure of the thermal neutron population, utilizes a measurement of the epithermal neutron population by means of two spaced neutron detectors each longitudinally spaced from a neutron source having a relatively high intensity neutron flux. Special detector means are utilized in the invention to effectively discriminate against the detection of thermal neutrons as proposed in the prior art. Moreover, by comparing the compensated porosity measurement using the dual speed detectors with an uncompensated porosity measured by the use of only one of the detectors, the location of borehole washouts or cement voids can be found.
In accordance with the invention, a relatively high intensity continuous neutron source irradiates earth formations surrounding a well borehole with a flux of neutrons. A pair of epithermal neutron detectors spaced longitudinally along the axis of the well tool from each other and from the neutron source measure the intensity of the epithermal neutrons at two different distances from the source. The ratio of the count rate occurring in each of the two spaced epithermal neutron detectors are then interpreted in terms of the earth formation porosity in accordance with a predetermined relationship. The porosity is also computed by the use of the count rate at the near detector alone. A comparison of the difference of the dual detector porosity and the single short spaced detector porosity can then be used to locate cement voids or washouts.
The apparatus of the present invention has been found to give improved results from prior art devices in that less sensitivity to disturbing environmental parameters results from its use in the dual detector compensated porosity measurement.